1. Field of the Invention
The present invention relates to a spindle motor for rotating magnetic discs, optical discs and the like, and a brushless motor having a structure similar to that of the spindle motor.
2. Description of the Related Art
In recent years, electronic equipment is becoming smaller and smaller. Thus, there is a growing demand for hard disc drives (HDDs) incorporated therein to become smaller and thinner. For example, until few years ago, HDDs for discs having a diameter of 3.5 inches or 2.5 inches had been used commonly. However, recently, HDDs for discs having a diameter of 1.8 inches or smaller, that is, 1 inch or 0.85 inches are becoming popular. When the disc diameter is reduced, motors such as spindle motors, brushless motors and the like for rotating the disc also have to become smaller and thinner. A reduction in size and thickness of motors is required not only in the field of HDDs but also in many other fields. As HDDs become faster, smaller and thinner, motors are required to rotate with a higher precision, lower vibration, and less noise. Thus, the type of a bearing used in motors is shifted from a conventional ball bearing to a hydrodynamic bearing. The hydrodynamic bearings float and rotate rotors without being in contact therewith by using a pumping force generated by fluid through a dynamic pressure generating groove. Therefore, it is necessary to pull the rotor to a stator in an axial direction with a certain force in order to prevent the rotor from moving in an axial direction and changing its position when a position of the motor is changed. In general, such an attractive force is obtained by two means.
The first means is a method of generating an attractive force between a rotor and a stator by utilizing a magnetic force applied by a rotor magnet to the stator by providing a magnetic member on a stator side of the motor or forming the stator side of the motor of a magnetic material.
The second means is a method of obtaining an attractive force in an axial direction by shifting magnetic centers of a rotor magnet and a stator core of the motor in the axial direction such that a magnetic force of the rotor magnet is generated to the stator side.
In general, the former means is used to obtain most of an attractive force and the latter means is used to achieve fine adjustment of the attractive force. If the attractive force is small, the rotor floats by a large amount. The rotor may float beyond a permissible limit of an upward movement in the bearing and come into contact with other components. An anti-vibration property of the whole device may also deteriorate. If the attractive force is large, the rotor floats by a small amount. When an amount of the oil is decreased in the later stage of the life of the bearing, the life may be shortened due to a bearing contact. In addition, vibration and noise of a spindle motor are also affected by the shift length of the magnetic centers.
Therefore, for a spindle motor employing a fluid bearing, an attractive force has to be adjusted and designed more precisely.
A brushless spindle motor, which is a first conventional example of a motor having a reduced size and thickness, will be described with reference to FIG. 13.
FIG. 13 is a cross-sectional view of the first conventional example, a brushless motor of an external rotor type described in Japanese Patent Gazette No. 3052540. In FIG. 13, ball bearings 106 are provided in a bearing support 104a in a central portion of a housing 104 having a plate-like shape. A shaft 101 with a rotor yoke 102 fixed thereto is fitted into a hole defined by inner ring 107 of the ball bearings 106. On an inner surface of the outer peripheral portion of the rotor yoke 102, a rotor magnet 103 is provided.
A stator core 108 of the stator is provided on the housing 104. The stator core 108 has projecting teeth portions (salient pole teeth portions) 108a in its outer peripheral portion as shown in a plain view of FIG. 14. The stator core 108 is attached to the housing 104 such that the projecting teeth portions 108a oppose the rotor magnet 103. The stator core 108 is formed by laminating multiple layers of thin magnetic plates punched into the shape shown in FIG. 14. A conducting wire is wound around salient pole arm portions 108b of the stator core 108 to form windings 109. In the stator core 108 of FIG. 14, winding 109 is formed for each of six salient pole arm portions 108b. The six windings 109 are connected as Y connection or Δ connection, for example, in the case of a three-phase motor. When a current flows through the windings 109, an electromagnetic force is generated between the stator core 108 and the rotor magnet 103, causing the rotor yoke 102 and the shaft 101 to rotate in a predetermined direction.
In order to obtain a brushless motor with a high torque, generally, it is necessary to increase a magnetic force of the rotor magnet 103, or the number of turns of the respective windings 109. However, in the structure of FIG. 13, the number of turns can be increased only to an extent that the lower end surfaces of the windings 109 come into contact with an insulating plate 105, which is a print substrate or the like provided on an upper surface of the housing 104. In such a state, a space 111 between the rotor yoke 102 and an upper surface of the windings 109 remains a spare space which is not fully utilized.
Japanese Patent Gazette No. 3052540 discloses a second conventional example obtained by improving the brushless motor shown in FIG. 13 to make full use of the space 111 between the rotor yoke 102 and the upper surfaces of the windings 109. FIG. 15A is a cross-sectional view of a brushless motor of the second conventional example. In FIG. 15A, the same elements as those shown in FIG. 13 are denoted by the same reference numerals, and the descriptions thereof are omitted.
In the brushless motor shown in FIG. 15A, the shape of a stator core 118 is different from that of the stator core 108 of the first conventional example. Specifically, the stator core 118 includes salient pole arm portions 118b and projecting teeth portions 118a which have the same planar shape as those of the salient pole arm portions 108b and the projecting teeth portions 108a of FIG. 14 but are different in that the projecting teeth portions 118a are bent downward in boundary portions between the salient pole arm portions 118b and the projecting teeth portions 118a as shown in FIG. 15. Since the projecting teeth portions 118a are bent downward, the entire stator core 118 can be moved away from the housing 104 in an upward direction and a gap between the lower end surface of the stator core 118 and the housing 104 can be made larger with the tips of the projecting teeth portions 118a facing the rotor magnet 103. With such a structure, the number of turns of the respective windings 119 can be larger than the number of turns of the windings 109 of the first conventional example. As a result, the space 111 between the stator core 118 and the rotor yoke 102 can be effectively used to achieve a stator having the windings 119 with large number of turns without changing the size of the entirety of the brushless motor.
The second conventional example of the brushless motor has following two problems.
The first problem is as follows. Since the projecting teeth portions (salient pole teeth portions) 118a of the stator core 118 are bent, the cross sections of the projecting teeth portions 118a of multiple core sheets forming the stator core 118 are not perpendicular but diagonal to the inner peripheral surface of the rotor magnet 103. When the projecting teeth portions 118a oppose to the rotor magnet 103 diagonally, an air gap between the rotor magnet 103 and the projecting teeth portions 118a expands substantially. Thus, a magnetic resistance is increased and an operational point of the rotor magnet 103 is lowered. Torque constant Kt (a value represented by a ratio of torque to current) is lowered.
Further, a position of the magnetic center of the stator core is difficult to be determined. A variance in the magnetic centers among the motors becomes large. Thus, there is large variance in attractive forces and motor properties such as an amount of floating and the life are not stabilized.
The second problem is due to a method for producing the stator. In order to produce the stator core 118 having the bent projecting teeth portions 118a, usually, core sheets are first laminated and then the laminated core sheets are collectively subjected to a bending working, or core sheets are first formed into a shape shown in FIG. 14 by a press working (punching), the projecting teeth portions 118a are bent by a bending working (process by a plastic deformation), and then the bent core sheets are laminated to produce the stator core 118. When the stator core 118 is produced by the former method, a large pressing force may be required depending upon the number of laminated layers. When the stator core 118 is produced by the latter method, the core sheets do not adhere closely to each other in the tip portions of the projecting teeth portions 118a, and small gaps 122 may be generated between the core sheets as shown in FIG. 15B. Thus, when the current flows through the windings 119, the projecting teeth portions 118a may vibrate, generating a noise. Furthermore, leakage flux from the stator core 118 increases and the magnetic property deteriorates. This increases power loss. It is also difficult to predict where the magnetic center of the stator core is, and a variance in the magnetic centers among the motors becomes large. Therefore, there is a large variance in attractive forces and the motor properties such as an amount of floating, the life, and the like are not stabilized.
The gaps between the projecting teeth portions 118a of the core sheets described above are generated for the following reasons. When the projecting teeth portions 118a of the plurality of core sheets are bent to form the stator core 118, for example, a flexural center C is set in a lower portion of the stator core 118 as shown in FIG. 16. Bent areas 120 of five core sheets 118g, 118h, 118i, 118j, and 118k are varied as shown in FIG. 16 with the largest being the bent area 120g of the core sheet 118g and the smallest being the bent area 120k of the core sheet 118k. If the core is formed as described above, no gap is formed between the laminated core sheets 118g through 118k. However, for varying the bent areas of the core sheets 118g through 118k, one bending mold is required for each of the core sheets 118g through 118k. If one mold is used for all the core sheets, a large strong bending working machine is required and a process cost is increased significantly.
Usually, the core sheets 118g through 118k are formed with one mold. Thus, the core sheets having the same bent area are laminated. FIG. 17 shows a laminate of a plurality of core sheets 125g through 125k formed with one mold. FIG. 17 is a partial cross-sectional view of laminated core sheets 125g, 125k, 125i, 125j, and 125k. As shown in FIG. 17, the core sheets 125g through 125k have planar portions 126 and bent portions 127. The bent portions 127 are formed by bending the right end portions of the core sheets 125g through 125k which have originally had a plate shape by, for example, thirty degrees with one mold. As shown in FIG. 17, when it is tried to laminate the core sheets 125g through 125k with the internal diameter sides of the planar portions 126 thereof being aligned and adhering closely to each other, the bent portions 127 of adjacent core sheets, for example, the core sheets 125g and 125h, overlap each other as shown by a shaded overlap portion 128. Actually, the core sheets 125g and 125h cannot overlap each other in the overlap portion 128. Thus, when the core sheets 125g and 125h are adhered closely to each other in the planar portions 126, they press each other in the bent portions 127 and a gap is generated in tip portions as shown in FIG. 15B. The same is also true of other core sheets 125i through 125k. 